1. Field of the Invention
The present invention relates to a process for perforating a stencil printing sheet. More particularly, the invention relates to a process for perforating a heat-sensitive stencil printing sheet in which excellent perforated images can be formed in a heat-sensitive stencil printing sheet.
2. Related Art
Stencil printing employs a stencil printing sheet (hereinafter may be referred to as stencil sheet) which is made either solely of a synthetic thermoplastic resin film (hereinafter may be referred to as resinous film) or in combination of a synthetic thermoplastic resin film and a porous support affixed thereto. Perforation of the stencil sheet is carried out, for instance, by first bringing image portions of a manuscript that contains a light-absorbing substance (usually, carbon black) into a close adhesion on the surface of the resinous film of stencil sheet, and irradiating infrared-rich rays from the sheet's side to generate heat in the image portions of the manuscript, thereby forming perforated images corresponding to the images of the manuscript. Alternatively, a plurality of heat-generating elements in a thermal head are contacted with the resinous film to selectively generate heat and to form perforated images corresponding to the images contained in the manuscript.
As mentioned above, according to these methods, perforations are formed by selectively melting a resinous film with the heat generated by the absorption of light energy in image portions of a manuscript, or with the heat generated in heat-generating elements. These perforation methods have the drawback that they tend to cause a perforation failure due to an adhesion failure between the resinous film and manuscript, or due to a contact failure between the resinous film and heat-generating elements. In addition, since the portions of the resinous film melted with heat is restrained by the manuscript or heat-generating elements, it cannot shrink back toward the periphery of perforations, partly deposit on heat-generating sections (image portions of the manuscript and heat-generating elements) as a melt, and thus causes the impediment of heat conduction. The melt, if not deposited onto heat-generating sections, stays in perforations as a melt residue to hinder the printing ink from flowing through the perforations during printing. As a result, clear images were hard to obtain.
Accordingly, an object of the present invention is to provide an improved process for perforating a stencil printing sheet in which excellent perforated images can be formed in a stencil printing sheet by using a thermal head, ink is smoothly passed through perforations, and clear printed images can be obtained.
As the results of extensive studies by the present inventors, it has been found that when the melted portions of the thermoplastic resin film is allowed to shrink back to the periphery of perforations during the formation of perforations in the resinous film with the heat from heat-generating elements of a thermal head, the stencil sheet is neatly perforated leaving substantially no melted resin in the perforations and without permitting deposition of the melted resin on heat-generating elements so that perforated images through which printing ink smoothly passes can be formed, and thus clear printing images can be obtained.